I am piping all e-mails through a PHP script that checks the To address against a database of valid addresses. If it exists, the rest of the script handles it. However, if it does not exist, how can I bounce the e-mail, the same way the server would if I didn't have the script? Thanks!
Turns out this was very easy to solve: Simply echo something in the PHP script (for example, "This account does not exist.") and the mailer daemon generates a bounce-back e-mail with this output included.